ANCHORAGE, AK, USA
N6951R
Beech B24R
The first pilot, a certificated flight instructor, was providing flight instruction to the second pilot, a certificated private pilot. While returning to the airport, during the initial descent, the engine began to run rough, and lose power. The roughness improved momentarily, followed by a severe engine vibration, a loud bang, and complete loss of engine power. During a forced landing in a densely populated neighborhood, the airplane struck a power line, veered to the right, collided with the street, and came to rest in the front yard of a daycare center. Examination of the engine revealed the number 3 cylinder was broken free at the crankcase-mounting flange. A metallurgical examination of the number 4 position crankcase thru-bolt showed signs of fatigue cracking that emanated from the thread root. In addition, the cylinder flange mating surface displayed signs of fretting damage consistent with relative movement between the cylinder and the crankcase. The operator's director of maintenance stated that about 40 hours before the accident, he repaired a substantial oil leak between the number 3 cylinder, and the engine crankcase. He said that in the process of the repair he removed the number 4 position crankcase thru-bolt nut, applied an engine sealant, reinstalled the through bolt nut, and torqued the nut in accordance with the Lycoming overhaul manual. There was no engine log book entry pertaining to the engine total time, aircraft total time, or date when this repair was accomplished.
HISTORY OF FLIGHT On May 9, 2000, about 1340 Alaska daylight time, a wheel equipped Beech B24R airplane, N6951R, sustained substantial damage during a forced landing about 1 mile east of Merrill Field, Anchorage, Alaska. The airplane was being operated as a visual flight rules (VFR) local area instructional flight under Title 14, CFR Part 91, when the accident occurred. The airplane was owned by Flight Safety Alaska, Inc., doing business as Take Flight Alaska. The first pilot, who was seated in the right seat and is a certificated flight instructor, and the second pilot, who was seated in the left seat, and is a certificated private pilot, were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight originated at Merrill Field, about 1220. During an on-scene interview with the National Transportation Safety Board investigator-in-charge on May 9, the first pilot reported that he was providing flight instruction to the second pilot, in preparation for his commercial pilot certification. He said that after completing the flight lesson for the day, he instructed the student pilot to return to the Merrill Field Airport for landing. The first pilot stated that during the initial descent to the airport, about 1,500 feet msl, the engine began to run rough, and lose power. He noted that the roughness improved momentarily, followed by a severe engine vibration, a loud bang, and complete loss of engine power. The first pilot selected a residential street situated within a densely populated neighborhood as a forced landing area. During approach, the airplane struck a power line, veered to the right, and collided with the street. The airplane came to rest in the front yard of a daycare center, and sustained substantial damage to the wings, fuselage, and empennage. There were no injuries of ground personnel. CREW INFORATION The first pilot, seated in the right seat, holds a commercial pilot certificate with airplane single-engine land, and instrument airplane ratings. In addition, the pilot holds a flight instructor certificate for single-engine land airplanes. His most recent first-class medical certificate was issued on October 15, 1999, and contained no limitations. According to the NTSB Pilot/Operator Report submitted by the operator on May 16, the pilot's total aeronautical experience consisted of about 523 hours, of which 45 were accrued in the accident airplane make and model. In the preceding 90 and 30 days prior to the accident, the report lists a total of 255 and 126 hours respectively. The second pilot, seated in the left seat, holds a private pilot certificate with airplane single-engine land, and instrument airplane ratings. His most recent second-class medical certificate was issued on May 1, 2000, and contained no limitations. According to the NTSB Pilot/Operator Report submitted by the operator, the pilot's total aeronautical experience consisted of about 453 hours, of which 77 were accrued in the accident airplane make and model. In the preceding 90 and 30 days prior to the accident, the report lists a total of 30 and 13 hours respectively. AIRCRAFT INFORMATION The airplane had accumulated a total time of 7,136.8 hours, and was maintained on a 100-hour/annual-inspection schedule. The most recent 100-hour inspection was accomplished on April 25, 2000, 41.0 hours before the accident. The engine, a Lycoming IO-360A1B6, had accrued a total time in service of 2,930.5 hours. The maintenance records note that a major overhaul was accomplished at the operator's maintenance department on October 10, 1999, 239.7 hours before the accident. According to the operator's engine maintenance records, the crankcase thru bolts were inspected and reused during the last engine overhaul. Prior to being reinstalled the crankcase thru bolts underwent a Magnaflux inspection preformed by Divco Inc., Tulsa, Oklahoma. During an on-scene conversation with the National Transportation Safety Board investigator-in-charge on May 9, the operator's director of maintenance stated that about 40 hours before the accident, he repaired a substantial oil leak between the number 3 cylinder and the engine crankcase. He commented that before the repair was accomplished, company pilots refused to fly the airplane until the oil leak was repaired. He said that in the process of the repair he removed the number 4 position crankcase thru-bolt nut, applied an engine sealant, reinstalled the thru-bolt nut, and torqued the nut in accordance with the Lycoming overhaul manual. On May 10, a Federal Aviation Administration airworthiness inspector from the Anchorage Flight Standards District Office, examined the airplane's engine maintenance records. He reported that the only reference to the number three cylinder oil leak repair was noted as item number 10 in the aircraft discrepancy log. The discrepancy notation read: "#3 cyc oil leak." In the action taken column it read: "Replaced base gasket seal through bolt, IAW MM." (In Accordance With Maintenance Manual) However, there was no reference pertaining to the engine total time, aircraft total time, or date when this repair was accomplished. WRECKAGE AND IMPACT INFORMATION The National Transportation Safety Board investigator-in-charge examined the airplane wreckage at the accident site on May 9. All of the airplane's major components were found at the main wreckage area. The right wing had leading edge aft crushing and upward curling of the wing tip. The left wing had extensive spanwise leading edge damage. The nose gear assembly separated from the fuselage. The airplane's lower keel area where the nose gear was attached, had upward crushing of the lower fuselage. The main landing gear was collapsed. The propeller assembly remained attached to the engine crankshaft. Both blades had minimal aft bending, leading edge gouging, and chordwise scratching. The flaps were in the fully extended position. Examination of the outside surface of the right side of the engine cowling, adjacent to the number 3 cylinder, revealed notable outward bulging to both upper and lower cowling assemblies. In addition, a substantial amount of oily and black residue extended aft, about 3 feet from the damaged cowling. Examination of the inside of the engine area revealed that the number 3 cylinder was broken free from the crankcase-mounting flange. The airplane was retrieved from the accident site and transported to the operator's hangar facility by company personnel located at Merrill Field. TESTS AND RESEARCH On May 10, the National Transportation Safety Board investigator-in-charge, and a Federal Aviation Administration airworthiness inspector from the Anchorage Flight Standards District Office, examined the airplane wreckage at the operator's facility. Disassembly of the airplane's engine cowling revealed a substantial amount of black, oily, residue on the inside of both upper and lower cowlings. The number 3 cylinder was broken free at the crankcase-mounting flange, and displaced downward, about 5 to 10 degrees. Examination of the interior and lower portion of the engine cowling revealed four, fractured 7/16-inch cylinder hold-down studs, along with the corresponding fractured hold-down nuts. Each fractured stud remained securely threaded into the nut. One, 1/2-inch crankcase thru-bolt nut, was discovered inside the lower portion of the engine cowling. A portion of the fractured thru-bolt was discovered still threaded into the nut. This nut and stud were later identified as being from the number 4 position of the number 3 cylinder assembly. The number 3 engine cylinder assembly, the fractured 1/2 inch crankcase thru-bolt, and the crankcase thru-bolt nut were sent to the National Transportation Safety Board's Materials Laboratory for examination. A Safety Board metallurgist reported that the fractured surface of the thru-bolt showed signs of fatigue cracking that emanated from the thread root. In addition, the cylinder flange-mating surface displayed signs of fretting damage consistent with relative movement between the cylinder and the crankcase. WRECKAGE RELEASE The Safety Board released the wreckage, located in Anchorage to the owner's representative on May 10, 2000. The number 3 cylinder, and crankcase thru-bolt and nut were retained by the Safety Board for examination until their release on September 13, 2000.
A fatigue failure of the crankcase through bolt, the separation of number 3 cylinder assembly, and the inadequate maintenance/service of the airplane by company maintenance personnel.
Source: NTSB Aviation Accident Database
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